The so-called thermite reaction traditionally involves the exothermic reduction of iron oxide with aluminum, in which the reaction produces molten iron with an aluminum oxide slag floating thereon, the reaction taking place either in a suitable mold so that the molten iron is fusion cast into a desired shape or at a site where two metal parts are to be joined to produce a weld between such metal parts when the reaction is completed.
Although there are prior patents which involve the use of the thermite type reaction to produce borides, carbides, silicides and nitrides and the like, the product produced by the reaction is of at least two phases, one which is a layer of the boride, carbide, etc., and another which is a layer of the oxide of the reducing metal such as aluminum or magnesium. That is, the reducing metal oxide is present as a separate layer of slag, as in the classical thermite reaction. If special steps are taken to produce a composition which is a mixture of the boride, carbide, etc. and the reducing metal oxide, such composition is not a foamed product.
UK Pat. No. 1,497,025 teaches the production of cast refractory inorganic products by a thermite type reaction in which slag is formed and the product is a dense, sintered form. Thus, the teaching of this patent is directed to producing a composition which is not a mixture, homogeneous or otherwise, of all the reaction products, but of a composition which is a mixture of the reaction products less the oxide of the reducing metal and (to the extent possible) less the CO which is formed during the reaction. This patent is specifically directed to avoid "poorly sintered specimens" of the desired product and to avoid products which are characterized by "porosity and the presence of free carbon therein, which affects their strength". To this end, the patent teaches a method which is carried out at a centrifugal acceleration of from 100 to 1,500 g and in a gaseous medium under pressure of 1 to 100 atm, using an inert gas such as argon. In this patent, the reaction mixture contains carbon and a reducing metal such as aluminum plus one or more metal oxides. The end product in each case is divided into two layers, a top layer of slag which is the reducing metal oxide and the bottom layer which is the desired material. Even if the contraints taught by this patent are not followed and porosity is present, it is not present in a composition which includes the reducing metal oxide.
Present techniques of producing refractory, monolithic shapes involve initial shape-forming steps such as hydraulic or isostatic pressing, slip-casting, extrusion, injection molding and the like prior to the firing step. Moreover, the firing step normally involves at least preheating the entire reaction mixture either to ignition temperature or to an elevated temperature at which local ingnition and subsequent completion of the reaction occurs.
The invention is directed to improvements over existing techniques.
In contrast to the prior art teachings, the invention herein relates to the discovery that precisely shaped, foamed, monolithic and thus highly insulative refractory articles may be produced in situ by controlling the particulate sizes of the reaction components which form the reactive mixture and the composition of the article being a substantially homogeneous dispersion of the products of reaction.
The invention relates to monolithic, shaped refractory articles which are of foamed nature yielding densities substantially less than the theoretical density of the composition of the article, and the method of making such articles.
This invention involves a powdered exothermic reaction mixture which may be loosely packed into a desired self sustaining shape and dimensions which, after local ignition and resultant reaction in air under ambient conditions, yields a foamed, monolithic article faithfully reproduced in the desired shape and dimensions.
The composition of the article is a substantially homogeneous mixture of the reaction products of the reaction mixture.
The foamed nature of the article, yielding a density less than that of the theoretical density of the composition of the article; and the ability of the reaction mixture to complete its reaction after local ignition in air under ambient conditions and to yield an article faithfully reproducing the shape and dimensions into which the reaction mixture has been loosely packed, are attained by controlling the particle sizes of the components of the reaction mixture and employing aluminum as the reducing metal of the exothermic reaction mixture.
The compositions of the articles of this invention are the reaction products of a thermite type reaction substantially homogeneously dispersed throughout the article and the article itself is characterized by its foamed nature and its faithful reproduction of the dimensions and shape into which the reaction mixture has been loosely packed.
The composition of the articles of this invention is preferred to be TiB.sub.2 in Al.sub.2 O.sub.3.
The reactive mixture of this invention is easily shaped by lightly packing it in a mold cavity defining the desired shape and dimensions of the article, the material defining the mold cavity being made of any suitable material which is capable of withstanding the temperatures involved during the reaction and which do not require great structural integrity other than to confine the lightly packed reactive mixture and retain the desired shape of the article before and during the reaction.
Moreover, the particulate size control renders the molded reactive mixture locally ignitable in air under atmospheric conditions without preheating the mass thereof, after which ignition the reaction proceeds to completion throughout the reaction mixture to produce the shaped article.
The method of this invention, then, is both energy and cost efficient and of minimal complexity, capable of allowing the production of shaped articles of highly refractory, insulating properties due not only to the composition of the article but also to the foamed nature thereof.
Insofar as the critical feature of particulate sizes of the components of the reaction mixture is concerned, I have found that all components must be of powder form to pass a screen of 50 mesh size, that is, not being retained on mesh size of larger openings. By mesh size is meant U.S. Standard mesh.
However, with respect to the ability of the reaction mixture to be ignited locally and thereafter react to completion in air under ambient conditions it has been found that the reaction mixture must contain a substantial amount of TiO.sub.2 which is of -300 mesh size, of B.sub.2 O.sub.3 which is of -100, +200 mesh size or less and of Al which is of -100, +200 mesh size or less.
The method of this invention does not require a special environment or other special preparation and/or reaction procedures such as preheating the reaction mixture mass, the use of controlled atmosphere during reaction, the application of centrifugal effect before or during reaction or the use of pressure to pre-form the reaction mixture. The "green" condition of the shaped reaction mixture prior to ignition is simply as a mass of powder which is loosely packed so as to attain the desired shape. Under these circumstances, ignition takes place in response to local heating in air at atmospheric pressure and may be effected by local heating as by a resistance heated nichrome wire until ignition occurs, whereafter the exothermic reaction progresses through the reaction mass until complete. A further advantage of the present invention is that the reaction achieves a less violent conversion of reactants to product than prior thermite reactions.
In accord with this invention, it is to be noted that the article formed is a composition derived from the two oxides which are present in the reaction mixture. Although it is preferred that this composition be that which results from stoichiometric amounts of these two components and of the reducing metal, it is not strictly necessary for successful compositions. For example, when titanium dioxide (TiO.sub.2) and boron oxide (B.sub.2 O.sub.3) are combined with aluminum in the reaction mixture, an excess of boron oxide will produce a composition which is titanium diboride (TiB.sub.2) plus the excess boron oxide plus aluminum oxide (Al.sub.2 O.sub.3) formed during the reaction. Similarly, if the reaction mixture were formed of stoichiometric amounts of the two oxides and an excess of the reactive metal aluminum, the product would be titanium diboride plus aluminum plus aluminum oxide.
The above and other objectives of the invention will become more apparent as the description proceeds.